1. Field of the Invention
The present invention relates to receiving apparatuses, and particularly to a receiving apparatus that performs reception processing of input packets sent in a burst.
2. Description of the Related Art
The proliferation of the Internet has spread the use of information communications networks among households and businesses in recent years. The progression of optical fiber subscriber networks toward higher-speed and higher-capacity services has encouraged the widespread adoption of a passive optical network (PON) system as an optical fiber subscriber system.
FIG. 14 shows the configuration of such a PON system 5. The PON system 5 includes an optical line terminal (OLT) 51 located on the side of a local exchange, optical network units (ONUs) #1 to #n disposed on the subscriber side, and a star coupler 52 for coupling and decoupling an optical signal.
The star coupler 52 makes a one-to-n connection between the OLT 51 and the ONUs #1 to #n with optical fiber cables F and allows optical packet communication between the OLT 51 and the ONUs #1 to #n. In the figure, the star coupler 52 couples optical packets sent in a burst from the ONUs #1 to #n of a plurality of subscribers, and the single OLT 51 receives those packets.
FIG. 15 is a view showing the levels of a signal received by the OLT 51. The horizontal axis represents time, and the vertical axis represents the level of the electrical signal after optical-electric conversion. The ONUs #1 to #n are placed in individual subscribers' homes. The transmission distance between the OLT 51 and the ONU varies among the ONUs #1 to #n, so the OLT 51 turns out to receive packet signals of different levels.
Each time a packet is received from each ONU, the OLT 51 specifies a threshold level for recognizing the code, judges whether the code is ‘1’ or ‘0’ in accordance with the threshold value, and performs data reception processing for the packet. In the shown example, the ONU #1 is the farthest from the OLT 51, and the ONU #5 is the closest.
In one conventional reception control technology (disclosed in Japanese Unexamined Patent Application Publication No. Hei-06-310967), a peak value and a bottom value detected from the input signal waveform are retained, a middle value between the two values is specified as a threshold value, and the input signal is amplified in a linear range having the threshold value at its center while a constant output amplitude is kept.
The OLT 51 includes a packet reception circuit. This circuit detects the peak voltage (voltage when the signal is high) and the bottom voltage (voltage when the signal is low) of a received packet and obtains a threshold value.
FIG. 16 is a view showing an overview of the packet reception processing. The horizontal axis represents time, and the vertical axis represents the signal level. Before the packet reception circuit receives a packet, the peak voltage, bottom voltage, and threshold voltage of the signal are reset. Then, the levels of peak voltage and bottom voltage of a received packet are detected from the level of the preamble (data indicating the position of the beginning of the packet) added to the first field of the received packet.
A midway level between the detected peak voltage and bottom voltage is obtained as the threshold voltage. After the reception of the packet ends, the peak voltage, bottom voltage, and threshold voltage are reset in preparation for the detection of the threshold value of a subsequent packet. This operation is performed for individual packets in the order in which they arrive, in the reception processing of a burst signal having input levels varying from packet to packet.
In the conventional packet reception circuit described above, even if the direct-current level does not vary during the input of the packet, the peak voltage and bottom voltage vary with the signal pattern of the input packet (data pattern of the packet), making it impossible to specify an accurate, constant threshold level.
FIG. 17 is a view illustrating the problem. The horizontal axis represents time, and the vertical axis represents the signal level. After the threshold level of a packet p1 is established, the detected levels are reset, and a packet p2 is received. As a first step for specifying a threshold level for the packet p2, the levels of the peak voltage and bottom voltage of the packet p2 are detected from the level of the preamble. Although the direct-current signal level of the input packet is constant during the input of the packet p2, the detected peak voltage and bottom voltage vary with the pattern of the input signal.
Since the threshold voltage depends on the varying peak voltage and bottom voltage (the figure shows that the peak voltage increases and decreases even though the input voltage of the packet p2 is constant), the threshold voltage varies while the same packet is being received. Therefore, a correct threshold value for judging the code cannot be specified.